According to the 2006 Food and Agriculture Organization of the United Nations (FAO) report Livestock's Long Shadow, industrialized agriculture contributes on a "massive scale" to global warming, air pollution, land degradation, energy use, deforestation, and biodiversity decline.[4] The FAO report estimates that the livestock (including poultry) sector (which provides draft animal power, leather, wool, milk, eggs, fertilizer, pharmaceuticals, etc., in addition to meat) contributes about 18 percent of global GHG emissions expressed as 100-year CO2 equivalents. This estimate was based on life-cycle analysis, including feed production, land use changes, etc., and used GWP (global warming potential) of 23 for methane and 296 for nitrous oxide, to convert emissions of these gases to 100-year CO2 equivalents. The FAO report concluded that "the livestock sector emerges as one of the top two or three most significant contributors to the most serious environmental problems, at every scale from local to global."[4]

Some sources[citation needed] disagree with some of the figures used in arriving at the FAO estimate of 18 percent. For example, the FAO report estimates that 37 percent of global anthropogenic methane emissions are attributable to the livestock sector, and a NASA summary indicates about 30 percent.[5] Because of the GWP multiplier used, such a difference between estimates will have a large effect on an estimate of GHG CO2 equivalents contributed by the livestock sector. Livestock sources (including enteric fermentation and manure) account for about 3.1 percent of US anthropogenic GHG emissions expressed as CO2 equivalents. This estimate is based on methodologies agreed to by the Conference of Parties of the United Nations Framework Convention on Climate Change.[6]

Impacts from agriculture are expected to increase substantially due to population growth and increasing consumption of animal products. Unlike fossil fuels, it is difficult to look for alternatives: people have to eat. A substantial reduction of impacts would only be possible with a substantial worldwide diet change, away from animal products.[7][8]

According to Cornell University scientists: "The heavy dependence on fossil energy suggests that the US food system, whether meat-based or plant-based, is not sustainable".[9] However, they also write: "The meat-based food system requires more energy, land, and water resources than the lactoovovegetarian diet. In this limited sense, the lactoovovegetarian diet is more sustainable than the average American meat-based diet."[9] One of these Cornell scientists has advised that the US could feed 800 million people with grain that livestock eat. He "depicted grain-fed livestock farming as a costly and nonsustainable way to produce animal protein", but "distinguished grain-fed meat production from pasture-raised livestock, calling cattle-grazing a more reasonable use of marginal land."[10]

According to a 2002 paper:

The industrial agriculture system consumes fossil fuel, water, and topsoil at unsustainable rates. It contributes to numerous forms of environmental degradation, including air and water pollution, soil depletion, diminishing biodiversity, and fish die-offs. Meat production contributes disproportionately to these problems, in part because feeding grain to livestock to produce meat—instead of feeding it directly to humans—involves a large energy loss, making animal agriculture more resource intensive than other forms of food production. [...] One personal act that can have a profound impact on these issues is reducing meat consumption. To produce 1 pound of feedlot beef requires about 2,400 gallons of water and 7 pounds of grain (42). Considering that the average American consumes 97 pounds of beef (and 273 pounds of meat in all) each year, even modest reductions in meat consumption in such a culture would substantially reduce the burden on our natural resources.[11]

Another agricultural effect is on land degradation. Cattle are a known cause for soil erosion through trampling of the ground and overgrazing.[13] Much of the world's crops are used to feed animals.[8] With 30 percent of the earth's land devoted to raising livestock,[14] a major cutback is needed to keep up with growing population. Demand for meat is expected to double by 2050; meat consumption is steadily rising in countries such as China that once followed more sustainable, vegetable-based diets.[citation needed]

The environmental impacts of animal production vary with the method of production. A grazing-based production can limit soil erosion and also allow farmers to control pest problems with less pesticides through rotating crops with grass. However, in arid areas, this may catalyze a desertification process. The ability of soil to absorb water by infiltration is important for minimizing runoff and soil erosion. Researchers in Iowa reported that a soil under perennial pasture grasses grazed by livestock was able to absorb far more water than the same kind of soil under two annual crops: corn and soybeans.[15] Corn and soybean crops commonly provide food for human consumption, biofuels, livestock feed, or some combination of these.

Massive reductions in meat consumption in industrial nations will ease the health care burden while improving public health; declining livestock herds will take pressure off rangelands and grainlands, allowing the agricultural resource base to rejuvenate. As populations grow, lowering meat consumption worldwide will allow more efficient use of declining per capita land and water resources, while at the same time making grain more affordable to the world's chronically hungry.[16]

Environmental vegetarianism can be compared with economic vegetarianism. An economic vegetarian is someone who practices vegetarianism either out of necessity or because of a conscious simple living strategy. Such a person may base this belief on a philosophical viewpoint, such as the belief that the consumption of meat is economically unsound or that vegetarianism will help improve public health and curb starvation.[17][18]

Environmental vegetarians call for a reduction of first world consumption of meat, especially in the US. According to the United Nations Population Fund, "Each U.S. citizen consumes an average of 260 lbs. of meat per year, the world's highest rate. That is about 1.5 times the industrial world average, three times the East Asian average, and 40 times the average in Bangladesh."[19] In addition, "the ecological footprint of an average person in a high-income country is about six times bigger than that of someone in a low-income country, and many more times bigger than in the least-developed countries."[20]

The World Health Organization calls malnutrition "the silent emergency", and says that it is a factor in at least half of the 10.4 million child deaths which occur every year.[21][22] Some[who?] argue that the adoption of an ovo-lacto vegetarian or entirely plant-based vegan diet is best, but may not be totally necessary, because even modest reductions in meat consumption in industrialized societies would substantially reduce the burden on natural resources. For developed countries, a CAST report estimates an average of 2.6 pounds of grain feed per pound of beef carcass meat produced. For developing countries, the estimate is 0.3 pounds per pound. (Some very dissimilar figures are sometimes seen; the CAST report discusses common sources of error and discrepancies among such figures.)[23] In 2007, US per capita beef consumption was 62.2 pounds per year, and US per capita meat (red meat plus fish plus poultry) consumption totaled 200.7 pounds (boneless trimmed weight basis).[24]

The support of alternative farming practices (e.g., well husbanded organic farming (as it is becoming usual in the European Union), permaculture, and managed intensive rotational grazing) and avoidance of certain plant commodities (e.g., rice) have a similarly beneficial impact on environmental health and sustainable agriculture.[25]

Bill Mollison has argued in his Permaculture Design Course that vegetarianism exacerbates soil erosion. This is because removing a plant from a field removes all the nutrients it obtained from the soil, while removing an animal leaves the field intact. On US farmland, much less soil erosion is associated with pastureland used for livestock grazing than with land used for production of crops.[26]Robert Hart has also developed forest gardening, which has since been adopted as a common permaculture design element, as a sustainable plant-based food production system.[27]

Some environmental activists claim that adopting a vegetarian diet may be a way of focusing on personal actions and righteous gestures rather than systemic change. Environmentalist Dave Riley states that "being meatless and guiltless seems seductively simple while environmental destruction rages around us", and notes that Mollison "insists that vegetarianism drives animals from the edible landscape so that their contribution to the food chain is lost."[28]